Nowadays, it has been desired to reduce emission of greenhouse gases such as carbon dioxide to prevent global warming and technologies are being developed for capturing carbon dioxide in an exhaust gas from an oxyfuel combustion boiler for storage in ocean or to underground formation.
A facility with such oxyfuel combustion boiler comprises mills for pulverizing coal, an air separation unit which separates oxygen from other nitrogen-rich gas, a forced draft fan (FDF) for air feed capable of forcing ambient air to inside of the system, a combustion furnace (boiler) which introduces, through an introducing line, the fuel from the mill and air from the forced draft fan or recirculation gas and oxygen from the air separation unit for combustion, an exhaust gas line which guides an exhaust gas from the combustion furnace to outside, an air preheater incorporated in the exhaust gas line, a dust collector for exhaust gas treatment incorporated in the exhaust gas line downstream of the air preheater and a recirculation line branched from the exhaust gas line and connected to the introduction line through the air preheater.
The combustion furnace has an inlet-side wind box which receives the air from the forced draft fan and the oxygen from the air separation unit. Arranged in the wind box is a burner which receives the pulverized coal from the mill.
For initial start-up of the facility with the oxyfuel combustion boiler, the burner is supplied with air through the introduction line, etc., to perform normal combustion with air, the resultant exhaust gas being entirely led to the exhaust gas line. In this case, the combustion with air will bring about the exhaust gas which contains about 70% of nitrogen and the rest comprising carbon dioxide, SOX and vapor; the exhaust gas is subjected to an exhaust gas treatment by the dust collector, etc., and emitted to the atmosphere with the components retained below environmental emission standard values. Subsequently, when collected heat of the combustion furnace reaches a designed value, a portion of the exhaust gas subjected to the exhaust gas treatment by the dust collector, etc., is recirculated as recirculation gas through the recirculation line and the oxygen from the air separation unit is mixed with the recirculation gas and fed to the wind box to perform combustion with the burner.
Thus, nitrogen contained in the air is no longer supplied and the concentration of nitrogen in the exhaust gas from the combustion furnace is gradually reduced as the concentration of carbon dioxide is increased. Subsequently, when the concentration of carbon dioxide becomes substantially constant, the emission to the atmosphere is terminated to perform steady operation while the exhaust gas is recirculated through the recirculation line and carbon dioxide is captured through a cooler as needed.
A facility using a boiler may include a seal as constructional detail on a top wall of the combustion furnace to prevent infiltration of combustion ash, etc. (see, for example, Patent Literatures 1 and 2).
[Patent Literature 1] JP 11-118102A
[Patent Literature 2] JP 2001-153303A